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The U.S. Air Force is investigating multiple instances of scrapes on aircraft caused during mid-air refueling performed by Boeing Co.’s new KC-46 tanker.

So far, the damage caused by the fuel probe that connects the tanker with other aircraft has been minor, according to a service statement and program documents. But it’s worrying enough that the Air Force issued a top-level “Category One” deficiency report on May 1 after it discovered the damage during post-test flight analysis of video and data.

The Air Force concerns center on whether the unintended contact could damage specialized coatings used on F-35 and F-22 stealth fighters and B-2 bombers, or cause structural damage, according to a program office assessment. There’s also the chance a KC-46 tanker would need to be grounded if the refueling probe was contaminated with stealth coating, it said.

While there is occasional contact between jets with refueling probes on current tankers, such incidents are happening more frequently now during test sorties with the new KC-46 tanker, Air Force spokeswoman Ann Stefanek told Bloomberg News.

“These contact events have caused surface scrapes on the coating just outside the receptacle and are not considered a mishap,” Stefanek said in a statement. “Further analysis is required to determine if” the contact jeopardizes meeting any key requirements, she said.

“The Air Force and Boeing are performing the necessary systems engineering analysis to determine the root cause,” Stefanek added. “The analysis will then be followed by the development of options to resolve the issue.”

$44.5 Billion

Stefanek said the Air Force is also assessing the potential additional impact, if any, to the current delivery schedule of the $44.5 billion acquisition program. The schedule has already slipped because of earlier technical problems, such as wiring. Those caused delays with a key milestone -- delivery of the first 18 tankers by this month. That is now expected by October 2018. Thirty-four aircraft are already on contract to be produced.

“We’re working with the Air Force to understand the concern,” Boeing spokeswoman Caroline Hutcheson said in an emailed statement. “This type of contact happens occasionally with all tanker aircraft, so we are trying to determine how the KC-46 rates compare with current fleet norms.”

“We’ve performed more than 1,000 boom connections to date on five different receiver aircraft,” Hutcheson added. “The testers have rated the controllability of the boom as very good, and we remain confident in the design of the boom handling system.”

Versatile Tanker

The KC-46 will carry about 212,000 pounds of fuel and is designed to resupply any U.S. warplane in midair. The wide-body plane, based on Boeing’s 767 jetliner, will be capable of carrying as many as 18 cargo pallets and performing medical evacuations of as many as 58 patients. It’s intended to replace the Air Force’s aging fleet of KC-135 tankers, also built by Boeing. The Air Force plans to initially buy 179 tankers with the potential for more.

Boeing is required to correct the issue with its own funds under its fixed priced-incentive fee contract, which caps the Air Force’s liability at $4.9 billion. The Air Force estimates Boeing will complete the contract for $6.3 billion, or $1.4 billion over budget. Boeing estimates it will complete the development work for $5.9 billion, the Air Force statement said.

The Air Force cost estimate “has remained constant for several years,” said the statement.

In air-to-air refueling, an operator maneuvers a hose, or probe, into the receiving aircraft’s fuel “receptacle.” The receptacle acts as a tray to guide the refueling probe and is designed for contact. But so-called “contact outside the receptacle” can damage the receiver aircraft.

“Multiple instances of contact outside the receptacle” were not recognized by the aerial refueling operators during missions, the program office said in an assessment to Pentagon officials.

Refueling Probe

Boeing’s KC-46 refueling system promises a major improvement over the aging KC-135 it is replacing. KC-135 operators have to lay on their bellies in a tail compartment in order to extend and guide the refueling probe into an aircraft receptacle with a joystick, looking through a window at the cruising aircraft as it maneuvers in flight to mate with the tanker.

The KC-46’s primary refueling feature that differs from the KC-135 is the “virtual boom operator capability,” said the Air Force. In that system, a KC-46 operator maneuvers and watches the probe via monitors at a station behind the pilots instead of through a tail window. The KC-46 operator executes refueling while peering through a 24-inch console that displays a 3D tanker picture.